Vat dye

Vat Dyes

1. VAT DYES

2. Properties Vat dyes are insoluble in water They are converted to their soluble “ leuco ” state by means of sodium hydrosulphite (reducing agent) in the presence of caustic soda “ Leuco ” compound have substantivity to cotton Vat dye have excellent washing and light fastness Vat dyes are expensive compared with the other classes of dye

3. Classification based on Method of Application IK Have optimum affinity at low temperature i.e. at 25 – 30 C Require minimum addition of caustic soda and sodium hydrosulphite IW Are exhausted at moderate temperature i.e. at 40 – 50 C Requires moderate caustic soda Requires moderate concentration of exhausting agent for better exhaustion IN Have maximum affinity for cellulose at 50 - 60 C Requires relatively high concentration of caustic soda in the dyeing bath The exhaustion of the dye bath is achieved even without the addition of common salt Require retarding agents during dyeing

4. Classification based on Method of Application IN-Special Require relatively high concentration of caustic soda, sodium hydrosulphite, Require high vatting and dyeing temperature Group Vatting Temperature ( C) Dyeing Temperature ( C) Caustic soda (g/l) Hydros (g/l) Common salt (g/l) IK 50 25-30 1.0 – 3.0 1.0 -5.0 6.0 – 50 IW 50 40 – 50 1.0 – 5.0 1.0 – 6.5 3.0 – 25 IN 50 – 60 50 – 60 1.5 – 10 1.5 – 10 None IN-Special Dyeing require special condition

5. Application/ Mechanism of Vat dyeing Vatting Conversion of insoluble commercial vat dye powder into its soluble sodium salt Dyeing Application of reduced and dissolved vat dye to the fibre from an alkaline reducing bath in presence of exhausting or retarding agent Oxidation The conversion of soluble salt of leuco vat dye absorbed by the fibre into insoluble form by aerial or chemical oxidation After treatment Treating with soap solution to get good fastness properties

6. O O ONa ONa Reduction Solubilisation Insoluble Vat Dye Soluble Vat Dye OH OH Oxidation [H] [O]

7. Vatting The conversion of insoluble vat dyes into their soluble form involves the following steps Reduction of insoluble vat dye into weakly acidic leuco form Neutralizing the leuco vat dye with sodium hydroxide to give water soluble sodium salt of leuco -vat dye The reduction followed by solubilising is called vatting To keep the dye in soluble sodium salt form, a sufficient excess of both caustic soda and hydrosulphite soda is required to be maintained in the bath Chance of sodium hydrosulphite to get decomposed to form acid In expose to air With increase in temperature

8. Dyeing This is the application of the dye to the textile material The get exhausted inside the textile material Have the following steps Adsorption Absorption or penetration

9. Oxidation The dyed goods are subjected to an oxidation for conversion of soluble sodium leuco-vat dye into insoluble vat dye Aerial oxidation Chemical oxidation (Hydrogen peroxide, sodium dichromate in presence of acetic acid etc.) The oxidation treatment has to carried out under mild condition (i.e. at low temperature and minimum time) to prevent oxidation of cellulose Due to over-oxidation change in tone may occur

10. After Treatment Dye particles in the residual liquor are also oxidized and get loosely deposited on the fabric surface These dye particles are not firmly fixed to the fibre and are therefore responsible for poor rubbing fastness properties During soaping the loosely held dye particles are removed and held in suspension to prevent them depositing back on the material The soaping treatment is carried out at boil The soaping treatment is done with soap and soda ash followed by washing treatment for 20 – 30 minutes

11. Dyeing Auxiliaries Reducing Agent The most important reducing agent for Vat dyeing is sodium hydrosulphite (Na 2 S 2 O 4 ) Alkali The most important is caustic soda (NaOH). Solubilising agent. Electrolyte Neutral salts e.g. common salt (NaCl) can increase the substantivity of leuco dyes for the fibre